Add span.subtype and span.action

[eyalkoren]

See elastic/apm#8
In DB and HTTP this means separating current dot-delimited span.types to the new attributes

[gregkalapos]

Very minor comment: I guess it should be "Add span.subtype". Just telling because I copy-pasted the issue title when created the corresponding .NET issue and I was asked about this :)

[felixbarny]

The Java agent will have troubles with that change. For the public API, the Transaction interface extends the Span interface -> a transaction is-a span and has more functionality: setUser and setResult and ensureParentId.

With this change Spans suddenly have capabilities a Transaction does not have. I'm not sure whether that's a good idea in general but when implemening this, I would rather split the type string for spans while serializing and keep the API setType(String) as-is.

Again, the problem is that if I add setType(String type, String sybtype, String action) to spans in the public API, the Transaction interface would inherit this method.

For the internal API, the situation is different and I could have different setType methods on co.elastic.apm.agent.impl.transaction.Transaction and co.elastic.apm.agent.impl.transaction.Span.

@roncohen could I get your input on this?